1. Field of the Invention
The present invention relates to an apparatus for providing temporary stability and floatation to marine or water borne structures (xe2x80x9cstructuresxe2x80x9d).
More particularly, the invention relates to: (1) a removable apparatus that can be temporarily attached to a structure such as a tension leg platform during the construction, transportation, installation and/or removal of the structure, where the device increases the stability of the structure by increasing its area at the water line; (2) structures having the apparatus(es) or modules attached thereto; and (3) method for using the modules during the construction, transportation, installation and/or removal of the structure.
2. Description of the Related Art
It is widely known and long established that significant and valuable natural resources are located on or beneath the ocean floor or other large bodies of water. This environment creates numerous obstacles or challenges to the exploration, mining or other collection of these resources.
Hydrocarbon liquids and gases trapped below the ocean floor are one of the most common and best known resources that are collected or mined. This mining and collection process has resulted in the construction of large offshore drilling, production, and utility platforms. Variations of platform design and construction have evolved. The earliest platforms were mounted on tall structures attached to the ocean floor. As the exploration of hydrocarbon fuels has progressed into deeper waters or more hostile environments, other platform designs have evolved, e.g., spar, single column floater (SCF) platform structures, and tension leg platform structures.
The cost and difficulty in constructing these structures are significantly diminished when the major portion of construction and assembly of the structure, the attached platform, ancillary equipment, and facilities, is accomplished at or near a shoreline fabrication site. In contrast, construction and assembly at or near the final ocean installation site, is often far removed from the necessary supply base and subject to inclement weather conditions.
Many common types of offshore platforms cannot be fabricated in their entirety at or near shore due to a variety of limitations. A spar platform typically has a large draft, which requires fairly deep water, i.e., greater than approximately 150 meters in its final vertical orientation. The spar platform is, therefore, generally transported on its side without associated equipment or ancillary facilities to the installation site. Offshore construction is required after the platform is upended to its vertical orientation to complete the facility. The cost of offshore construction is substantially greater than construction at an onshore facility. Alternate, devices or methods require the structure to be tilted along its vertical axis to control the structure""s stability during installation.
Tension leg platforms (TLPs) can and have been fabricated in their entirety at or near shore and towed as a complete platform to the installation site. However, the efficiency of the platform is compromised because the structure must be designed to be satisfactorily stable at a much shallower draft than the design installed draft. Adequate stability requires larger columns or wider column spacing than would be required for the operation of the structure after installation. Both features, i.e., construction of larger columns or placement of columns at wider intervals, add significant costs to the structure.
Recent advancements in tension leg platforms include single column and extended base varieties. A complete single column tension leg platform, including the platform, deck, equipment and related facilities cannot be constructed at or near shore, because the structure is not stable about its vertical axis until after tendon installation is complete. If the structure is constructed on its side at a shore fabrication site, the ancillary deck, equipment and facilities cannot be constructed until the structure is righted, tendons attached and installation completed. Moreover, these alternate designs for tension leg platform are more efficient when designed and constructed to provide stability only after tendons attachment.
Many devices and techniques have been described in the prior art for transporting structures to an offshore installation site. Many have related to the placement of the structure on its side and floating it to the site. The structure can then be then placed in the final upright position by various techniques such as controlled flooding of the structure or removal of floatation devices. Other devices or techniques have utilized the tipping of the structure during the installation process in order to facilitate stability during installation. Examples of such prior art are found in U.S. Pat. Nos. 3,811,681, 3,823,564, 3,859,804, 3,868,886, 4,062,313, 4,112,697, 4,385,578, 4,648,751, 4,768,456, 4,809,636, 4,811,681, 4,874,269, 4,913,591, 5,224,962, 5,403,124,5, 524,011, and 5,924,822, incorporated herein by reference. However, these devices and/or techniques do not permit the structure, including but not limited to the ancillary platform, deck, equipment and other facilities to be constructed in its final installation orientation, transported to the installation site and installed and secured without tipping the structure or permanently incorporating additional physical elements into the structure that permit such construction, transportation and installation.
Thus there is a need in the art for a device and method that allows the structure to be constructed, transported, installed and later removed in a substantially upright orientation.
Modules
The present invention provides an apparatus or module adapted to increase stability and optionally floatation of offshore structures, such as tension leg platforms (TLPs), where the module is removable so that its can be temporarily attached to the structure during structure construction, transportation, installation and/or removal. The present invention provides an apparatus or module adapted to increase stability and optionally floatation of offshore structures, such as TLPs, where the module is removable so that it can be temporarily attached to the structure during structure construction, transportation, installation and/or removal and the modules can be hollow, solid, rigid, semirigid, and/or flexible, and can be constructed to be ballasted or deballasted.
Offshore Structures with Modules Attached
The present invention also provides an offshore structure including a module to increase structure stability and optionally floatation to maintain the structure in a substantially upright orientation during structure construction, transportation, installation and/or removal, where the module is removable so that it can be temporarily attached to the structure and at least a portion of the module extends above a waterline (above a surface of a body of water in which the structure is installed).
The present invention also provides an offshore structure including a plurality of removable modules temporarily attached to and disposed at different locations on the structure, where the modules are adapted to increase structure stability and optionally floatation and at least a portion of each module extends above a waterline.
The present invention also provides an offshore structure including a plurality of removable modules attached to and symmetrically disposed about a central vertical axis of the structure, where the modules are adapted to increase structure stability and optionally floatation and at least a portion of each module extends above a waterline.
Although offshore structures such as TLPs generally having buoyant and ballast (floodable) compartments, the inclusion of ballasting compartments adds considerable expense to manufacture and upkeep of the structures because the floodable compartments must be resistant to corrosion and must have valving so that water and/or air can be pumped into or out of the compartments. The modules of the present invention can actually be used to eliminate the need for ballast compartments on the structure itself. Thus, the temporary modules can include all the equipment needed to change the draft of the structure including increasing the draft of the structure (lower the structure in the water) to permit or facilitate tendon attachment to a lower portion of the structure and all tensioning of the structure after tendon attachment.
Methods
Attaching
The present invention also provides a method for increasing the stability of an offshore structure including the steps of attaching one or more modules of this invention to the structure so that at least a portion of each module extends above a water line, where the modules increase a structural stability of the structure.
The present invention also provides a method for increasing the stability of an offshore structure including the steps of attaching a number of modules of this invention to the structure so that at least a portion of each module extends above a water line, where the number of modules is sufficient to maintain the structure in a substantially upright orientation.
Attaching and Removing
The present invention also provides a method for increasing the stability of an offshore structure including the steps of attaching one or more modules of this invention to the structure so that at least a portion of each module extends above a water line, where the modules increase a structural stability of the structure and removing the modules from the structure.
The present invention also provides a method for increasing the stability of an offshore structure including the steps of attaching a number of modules of this invention to the structure so that at least a portion of each module extends above a water line, where the number of modules is sufficient to maintain the structure in a substantially upright orientation and removing the modules from the structure.
Attaching, Changing
The present invention also provides a method for increasing the stability of an offshore structure including the steps of attaching one or more modules of this invention to the structure so that at least a portion of each module extends above a water line and changing a ballast state of at least one of the modules to change the stability of the structure and/or to change a draft of the structure, while maintaining the structure in a substantially upright orientation.
The present invention also provides a method for increasing the stability of an offshore structure including the steps of attaching a number of modules of this invention to the structure so that at least a portion of each module extends above a water line, where the number of modules is sufficient to maintain the structure in a substantially upright orientation and changing a ballast state of at least one of the modules to change the stability of the structure and/or to change a draft of the structure, while maintaining the structure in a substantially upright orientation.
Attaching, Changing, Removing
The present invention also provides a method for increasing the stability of an offshore structure including the steps of attaching one or more modules of this invention to the structure so that at least a portion of each module extends above a water line, changing a ballast state of at least one of the modules to change the stability of the structure and/or to change a draft of the structure, while maintaining the structure in a substantially upright orientation and removing the modules from the structure.
The present invention also provides a method for increasing the stability of an offshore structure including the steps of attaching a number of modules of this invention to the structure so that at least a portion of each module extends above a water line, where the number of modules is sufficient to maintain the structure in a substantially upright orientation, changing a ballast state of at least one of the modules to change the stability of the structure and/or to change a draft of the structure, while maintaining the structure in a substantially upright orientation and removing the modules from the structure.
Attaching, Transporting, Changing, Installing and Removing
The present invention also provides a method for increasing the stability of an offshore structure including the steps of attaching one or more modules of this invention to the structure so that at least a portion of each module extends above a water line, transporting the structure with attached modules from a first site to a second site, changing a ballast state of at least one of the modules to change a draft of the structure, while maintaining the structure in a substantially upright orientation so that a bottom portion of the structure is submerged to a sufficient depth to permit attachment of anchoring tendons, attaching a plurality of tendons to the bottom portion of the structure, deballasting the modules to remove water from an interior of each modules and removing the modules from the structure.
The present invention also provides a method for increasing the stability of an offshore structure including the steps of attaching a number of modules of this invention to the structure so that at least a portion of each module extends above a water line, where the number of modules is sufficient to maintain the structure in a substantially upright orientation, transporting the structure with attached modules from a first site to a second site, changing a ballast state of at least one of the modules to change a draft of the structure, while maintaining the structure in a substantially upright orientation so that a bottom portion of the structure is submerged to a sufficient depth to permit attachment of anchoring tendons, attaching a plurality of tendons to the bottom portion of the structure, deballasting the modules to remove water from an interior of each modules and removing the modules from the structure.
These last two methods can also include step of changing the ballast of one or more of the modules to increase, maintain or decrease structure stability and/or draft during the transporting or tendon attaching steps.
Other variations, changes or modification of the invention will be recognized by individuals skilled in the art that do not depart from the scope and spirit of the invention described and claimed herein.